1. Field of the Invention
The present invention relates to an apparatus for driving a liquid crystal display (LCD) apparatus, and more particularly, to a driver (buffer) of the LCD driving apparatus.
2. Description of the Related Art
Since LCD panels are thinner in size and lower in power dissipation as compared with cathode-ray tube (CRT) panels, the LCD panels have recently been applied to personal computers, a word processors, color telereceivers. Particularly, since active matrix-type LCD apparatuses have high speed response, a fine screen with a high quality, and a multi-gradation display, the active matrix-type LCD apparatuses have been in demand.
Generally, an active matrix-type LCD apparatus is constructed by a semiconductor substrate having thin film metal wire, a transparent pixel electrodes and thin-film transistors (TFTs), a counter substrate having a transparent common electrode, and liquid crystal inserted between the semiconductor substrate and the counter substrate. A gradation voltage is applied to each pixel electrode by controlling the TFT with a switching function, and transmittance of the liquid crystal is changed by the difference in voltage between each pixel electrode and the common electrode to provide display on the sereen.
Provided on the semiconductor substrate are data lines for applying gradation voltages to the pixel electrodes and scan lines for applying switching control signals (scan signals) to the TFTs. Then, when the scan signal of the scan line is at a high level, all the TFTs connecting the scan line are turned ON, and the gradation voltages sent to the data line are applied to the pixel electrodes through the TFTs. When the scan signal becomes low to turn OFF the TFTs, the difference in voltage between each pixel electrode and the common electrode is maintained until the next gradation voltages are applied to the pixel electrodes. Thus, when scan signals are sequentially sent to each scan line, gradation voltages are applied to all the pixel electrodes, so that display on the screen is renewed at every frame period.
An LCD driving apparatus for driving the data lines is required to charge/discharge a large load of each data line including a liquid crystal capacity, wiring resistances and wiring capacities.
An LCD driving apparatus is generally constructed by a voltage divider, a decoder and driver connected to a data line. A prior art driver is formed by an operational amplifier (see: S. Saito et al., "A 6-bit Digital Data Printer for Color TFT-LCDs", SID 95 Digest, pp. 257-260, 1995). Since the operational amplifier has a high current supplying capability, the driver can drive the data line having a large capacity at a high speed. Additionally, even when the threshold voltages of transistors within the operational amplifier fluctuate slightly, the fluctuation of the output voltage of the operational amplifier is relatively small. Further, the output voltage can be highly accurate. This will be explained later in detail.
In the prior art driver, however, if the LCD driving apparatus is constructed by a single integrated circuit device, the number of operational amplifiers with a large number of elements is increased as the number of data lines is increased. Therefore, the chip size is increased which increases the manufacturing cost. In addition, steady currents are required for the operational amplifiers, which increases the power dissipation.